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American bittern

Eye placement is a matter of survival

The wind blows over the marsh on a cool afternoon, and the reeds sway gently in the breeze. I’ll bet you never saw the American bittern concealed among the vegetation. I’m quite sure I have missed many.

Standing bolt upright with bill pointed to the sky, bitterns are nearly invisible. The vertical brown stripes on the breast blend nearly perfectly with the vegetation. To complete the deceit, the bird even rocks back and forth slightly as the reeds rustle.

There are many exquisite examples of camouflage in nature, including walking stick insects, patterned snakes coiled among the leaves on the forest floor and stonefish that look like anything but a fish as they wait for an unsuspecting meal to swim by. However, I present the story of the bittern to explore a completely different issue: Where is it best to have eyes placed on your head?

Our eyes are in front, side by side. Having both eyes that face forward allows for a large overlapping field of vision. With both eyes focused on an object, our brain can measure the angles involved and calculate the distance to the object quite accurately. This is helpful when driving a nail with a hammer or catching a ground ball in baseball.

It is also most helpful for predatory birds such as eagles, falcons and owls when attempting to capture prey. The only drawback is that these birds cannot see behind them when looking forward. This isn’t so bad when you are at the top of the food chain. The eagle seldom has to worry about another animal sneaking up from behind.

Good binocular vision isn’t for every bird. A pigeon, for example, has less need for binocular vision. Seeds on the ground don’t move much, and the distance is pretty much fixed by the length of their legs.

However, many predators consider squab a delicacy. Here a different strategy for the eyes is needed. It’s better for a pigeon to have eyes on the sides of its head. This allows for 360-degree vision. In this way a pigeon fleeing a falcon can see where it is going in front and still watch the rapidly approaching predator from the rear. I have watched a peregrine falcon overtake a fleeing pigeon at incredible speed. At the very last instant the pigeon, with its bill pointed forward, swerved sharply causing the falcon to miss. Because it could see directly behind, it knew exactly when to execute the evasive maneuver ... and survive. I’ve often wondered if the same arrangement of eyes explains the exceptional vision of teachers.

OK, what does any of this have to do with a bittern rocking in the breeze? Well, herons, egrets and bitterns hunt fish and other swimmers. These are at their feet in the water. It makes sense that their eyes are directed downward, making it easier to locate their prey. So when a bittern points its bill skyward, not only is it mimicking the reeds, it is gaining a better look at a potential threat. The eyes that usually point downward are now pointed at you and with binocular vision.

Stewart Janes is a biology professor at Southern Oregon University. He can be reached at janes@sou.edu.

 

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